A wireless device that receives a radio signal transmitted by a communication device and outputs information for guiding the wireless device based on the received radio signal is known. As an example of this type of wireless device, a mobile communication terminal disclosed in Patent Literature 1 (Japanese Laid-open Patent Publication No. 2001-36320) measures signal strength in each direction using a directional antenna. The mobile communication terminal outputs information for guiding the wireless device to such a position that a stronger radio signal is received from a base station based on the measured signal strength.
Note that Patent Literature 2 (Japanese Laid-open Patent Publication No. 2008-252709) discloses a technique of using a spatial correlation value. Specifically, the base station disclosed in Patent Literature 2 calculates a spatial correlation value based on radio signals transmitted from a plurality of terminal devices, determines whether the calculated spatial correlation value is equal to or larger than a threshold value, and determines whether the base station can execute radio communication with the plurality of terminal devices according to a space division multiple access (SDMA) scheme based on the determination result.
However, a radio communication system (for example, a mobile communication system, a radio sensor network, and the like) that includes a plurality of communication devices that perform radio communication with each other is known. In this type of radio communication system, an abnormality may be detected in the radio communication between first and second communication devices.
In this case, it is ideal that a wireless device is disposed at such a position that the wireless device can receive a radio signal having sufficiently strong correlation with a radio signal that is transmitted by the second communication device and is received by the first communication device, and the wireless device examines the quality of the radio signal. Thus, the technique disclosed in Patent Literature 1 can be applied. According to this technique, it is possible to guide the wireless device to be located sufficiently near the first communication device.
However, as illustrated in FIG. 1, the radio signal transmitted by a second communication device R2 may be blocked by obstacles and/or the geography. In this case, even when a wireless device S1 is located sufficiently near a first communication device R1, there is a problem in that it is difficult for the wireless device S1 to receive a radio signal having sufficiently strong correlation with the radio signal that is transmitted by the second communication device R2 and is received by the first communication device R1.